This invention relates to side pincushion distortion correction circuits suitable for use in television receivers.
Pincushion distortion of a raster formed on the viewing screen of a cathode ray tube, such as the picture tube utilized in television receivers, is manifested by a bowing outwardly of the edges of the raster. This distortion is caused primarily by the curvature of the viewing screen. It is well known that side pincushion distortion may be reduced by modulating the horizontal rate scanning current flowing through the horizontal deflection coils at the vertical deflection rate. Usually the modulation is parabolic at the vertical rate so as to reduce the horizontal scan at the top and bottom portions relative to the horizontal center line of the raster. Known arrangements for effecting such modulation include placing secondary windings of a saturable reactor or transformer in circuit with the horizontal deflection coils and energizing the primary or control winding by a suitable parabolic vertical rate signal, or, directly controlling the amplitude of the scanning current by biasing the horizontal deflection stage or altering its operating voltage level at the vertical rate.
One arrangement for effecting side pincushion distortion correction is disclosed in U.S. application Ser. No. 722,600 filed Sept. 13, 1976 by Peter Eduard Haferl and entitled "Pincushion Correction Circuit" in which an inductance in series with the horizontal deflection coils is shunted by the series arrangement of a switch and a resonant circuit. The longer the switch is closed the greater the horizontal scanning current. The switch is controlled by a modulation in which a parabolic vertical rate signal is combined with linear, sawtooth-shaped horizontal rate signals for producing a train of horizontal rate signals pulse-width modulated at the vertical rate. The modulation is parabolic and results in a parabolic correction of the pincushion distortion.
However, it has been determined that, depending on such factors as the geometry of the picture tube, a horizontal scanning current modulation other than parabolic may be required to minimize side pincushion distortion.